There is a little-noticed downside to Microsoft Corp.'s investment in Facebook Inc.: The deal will likely raise the price of stock options issued by the social-networking company and could make it more difficult to hire talented employees.

Last week, Facebook appeared to score a major victory when Microsoft said it would invest $240 million in the Palo Alto, Calif., start-up, in exchange for a 1.6% stake. The investment values Facebook at $15 billion, up significantly from last year when a financing round valued the company at $525 million, according to a person familiar with the matter. Microsoft's investment cemented Facebook's reputation as one of the hottest Web start-ups in Silicon Valley.

With the rise in valuation comes a rise in the value of employee stock options. And in Silicon Valley, where stock options can be a major component of employee pay packages, more expensive stock options mean less potential upside for the option holders once start-ups go public or are sold.

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